1. Field of the Invention
The present invention generally relates to charging amount display apparatus and, more particularly, is directed to an apparatus for displaying a charge amount of a re-chargeable battery, such as a lead storage battery or the like.
2. Description of the Prior Art
Conventionally, in a charging apparatus for charging a secondary battery, such as a lead storage battery, a charging current of the secondary battery is detected and a charging amount is displayed in order to display the charged condition of the secondary battery. However, in such secondary battery, as shown by a charging voltage characteristic curve 20 in FIG. 1, after a predetermined period of time, the charging voltage is placed in a saturated condition 20a before the charging amount reaches to the fully charged state (100%) as shown by a charging amount characteristic curve 21 so that, even when the charging voltage is detected, if the charging voltage is less than a predetermined voltage, then the charging condition cannot be displayed. That is, in the ordinary charging apparatus, only about 70 to 80% of the charging condition is displayed. To solve this problem, as shown in FIG. 1, when the charging voltage approaches the saturated condition 20a, a charging current characteristic curve 22 which is constant up to that time begins to decrease, and for this reason, a decreasing charging current 22a is detected in order to display the charging condition. FIG. 2 shows an example of an arrangement of a conventional charging amount display apparatus.
Referring to FIG. 2, an AC power source voltage from an AC plug 1 which is inserted into a commercially available AC power supply source is filtered out by a filter section 2 and fed to a switching section 3. A switch timing of the switching section 3 is controlled by a control signal which results from converting control data of a microcomputer (hereinafter referred to as a central processing unit (CPU)) 10A into an analog signal. A switching signal, which is controlled to have a predetermined pulse width by the switching section 3, is supplied to a primary side of a conversion transformer 4, and an increasing voltage (decreasing voltage) conversion signal developed at the secondary side of the conversion transformer 4 is rectified and smoothed by a rectifying and smoothing circuit 5, converted into a DC voltage and is then fed to a positive terminal 13 to which an anode terminal of a secondary battery 12 to be charged is connected. A negative terminal 14 is connected with a cathode terminal of the secondary battery 12 and also with one end of a charging current detection resistor 15 and the other end of this resistor 15 is grounded. A charging voltage detecting circuit 19 is connected to the positive terminal 13 while a charging current detecting circuit 18 is connected to the negative terminal 14, whereby the charging voltage and the charging current detected by the two detecting circuits 19 and 18 are converted to digital data by analog-to-digital (A/D) converter circuits 17 and 16 respectively and charging voltage data and charging current data are supplied to the CPU 10A. The CPU 10A generates data corresponding to the charging voltage or charging current and supplies the same to similarly two digital-to-analog (D/A) converter circuits not shown. By outputs of these D/A converter circuits, the switching section 3 is controlled and a charging state is displayed on a display section 11.
According to the prior-art charging amount display apparatus shown in FIG. 2, when the secondary battery 12 is charged, during the charging voltage characteristic curve 20 shown in FIG. 1 is increased, the charging voltage is detected by the charging voltage detecting circuit 19 to thereby control the switching section 3. Under the condition that the charging voltage becomes constant, the charging current 22a in which the charging current characteristic curve 22 shown in FIG. 1 is decreased is detected by the charging current detecting circuit 18 to thereby control the switching section 3. As a consequence, when the switching section 3 is controlled by the CPU 10A or the like, not only the two A/D converter circuits 16 and 17 and the two D/A converter circuits are needed but also the charging current detecting circuit 18 and the charging voltage detecting circuit 19 are needed, which provides a complicated circuit arrangement of the charging amount display apparatus.
Further, in a re-chargeable secondary battery and so on, when a charging amount in the charging-process of the secondary battery is measured, if a correlation shown by a characteristic curve 30 in FIG. 3 is established between a secondary battery release voltage (V) and a charging amount (%), then the charging amount of the secondary battery can be detected by measuring release voltages V1, V2, . . . , V4 of the secondary battery. As shown in a circuit diagram of FIG. 4A, when a secondary battery 42 to be charged is connected through a switching means 32 to an output terminal of a battery charger 31 having a plug 41 which is inserted into an commercially available AC power source and a release voltage of the secondary battery 42 in the charging-process is measured, the switching means 32 is turned OFF and the release voltage of the secondary battery 42 is measured by using a voltmeter 33 or the like. Generally, when the secondary battery 42 is rapidly charged, a constant current charging is performed and a charging current of, for example, a relatively large current value of 1A to 2A is supplied to the secondary battery 42. Therefore, a large current switch is required as the switching means 32 which turns ON and OFF this large current, and such large current switch increases a manufacturing cost of the charging amount display apparatus and also deteriorates reliability thereof.
Furthermore, a voltage drop occurs due to an interconnection resistance and a contact resistance between the battery charger 31 and the secondary battery 42, and an inside resistance of the secondary battery 42, when it is rapidly charged by the large current. Particularly, in a video camera or the like in which a secondary battery is housed in a housing and treated as one of video camera accessories, positive and negative polarity terminals of secondary battery in the mounting and/or dismounting process are formed as contact structures, and the secondary battery is connected to the battery charger 31 by means of the contact structures. Although a contact resistance of that contact structure is usually in a range of from about 50 m.OMEGA. to 100 m.OMEGA., if the secondary battery is connected to and disconnected from the battery charger repeatedly, the contact resistance is changed to about 200 m.OMEGA.. More specifically, the contact resistance is changed each time the secondary battery is connected to and disconnected from the battery charger, and the amount of the change of release voltage of the secondary battery in the charging-process is 0.4 V if the charging current is 2 A and the contact resistance is 200 m.OMEGA.. If the charging current is 2 A and the contact resistance is 100 m.OMEGA., the amount of the change of the release voltage is 0.2 V, and thus, the amount of the change of the voltage is increased. More precisely, as shown by a graph of charging characteristic curves of FIG. 5, a difference of voltage occurs between a battery voltage characteristic curve 45 in the rapid charging process and a secondary battery release voltage characteristic curve 46 as shown by a broken line 46. In FIG. 5, reference numeral 47 designates a charging current characteristic curve of the rapid charging process. If the release voltage is detected by the aforementioned method of FIG. 4A without being affected by the change of the voltage of the release voltage as described above, then unavoidably the switching means 32 of large capacity must be provided to cause the earlier noted disadvantages.
To solve such problem, as shown in FIG. 4B, a plurality of contacts 43, 44, 43a and 44a are provided in a housing 48 which houses the secondary battery 42, and the secondary battery 42 is connected between the contacts 43 and 44 and the voltmeter 33 or the like is connected to the contacts 43a and 44a to thereby detect the voltage.
As described above in the examples of the prior art, the charging apparatus shown in FIG. 4A needs the switching means 32 of large capacity to turn ON and OFF the current of large current value 1A to 2A in the rapid charging processing, and the switching means 32 which can endure such large current is not only expensive but also low in reliability. Furthermore, the charging apparatus of FIG. 4B is disadvantageous in space factor and in cost, and also, the voltage is detected while the inner resistance of the secondary battery is involved, which deteriorates accuracy in measuring the release voltage of the secondary battery.